1. Field of Invention
This invention relates to aircraft, specifically to a novel flight control system for enabling the pilot to transmit flight control signals from the flight deck to the aircraft's directional control equipment, even in the event of a control system failure.
2. Description of Prior Art
In the early days of aviation, pilot-generated flight control forces for controlling the aircraft's flight direction were transmitted from a hand-operated joystick and rudder foot pedals. The joystick is a vertical rod gimballed at the bottom, so that the pilot could move its top in any direction. The joystick was connected to exterior flight control surfaces, through pulleys and cables, positioned inside the fuselage and wings. The cables went through the interior of the fuselage and wing structure to moveable control surfaces (flap type structures) located on the trailing edges of the wing and horizontal stabilizer (rear wing). These moveable control surfaces on the trailing edges of the wing are called ailerons, while moveable control surfaces on the trailing edges of the horizontal stabilizer (rear "wing") are called elevators.
E.g., when the pilot pushed the joystick to the right, it raised the trailing edge of the right aileron and the airstream pushed the right wing down. At the same time the right aileron was raised, lowering the right wing, the left aileron was lowered and the left wing was thereby raised. This caused the aircraft to roll to the right around the horizontal axis of the fuselage.
When the pilot pulled back on the joystick, cables located within the fuselage raised the trailing edge of the elevator control surface. The airstream pushed the tail down, causing the nose to rise around the transverse horizontal axis from wingtip to wingtip.
The pilot also used foot pedals that were connected by cables to a hinged moveable control surface located at the trailing edge of the vertical tail surface (rudder). When the pilot pushed the right foot pedal it moved the trailing edge of the rudder surface to the right, whereupon the airstream pushed the tail of the aircraft to the left, causing the nose to move to the right around the vertical axis of the aircraft.
These movements of flight control surfaces are the same today as they were in the early days of aviation.
To control the engine, the pilot used a throttle in the early days as well as today. The throttle was and still is connected to the engine by cables, rods and tubing.
At the present time, in many commercial and military aircraft, pilot-generated forces are transmitted through small tubes located within the fuselage. These tubes contain hydraulic fluid, under pressure, which is used to activate the external wing and tail flight control surfaces and engines.
Recently, a new technology has developed which uses transducers to convert pilot-generated mechanical forces to electronic signals. These electronic signals are then sent to a flight computer for modification. The computer produces digital control signals which are then transmitted, via internal wiring, located within the fuselage and wing structures, to the external moveable flight control surfaces. This is known as a "fly-by-wire" mode.
On a program titled "Next Step", on Sunday, Nov. 13, 1994 at 6:30 PM on San Francisco television Channel 4 (KRON), the construction of Boeing Aircraft's new model 777 aircraft was discussed and shown. One feature shown was the new primary "fly-by-wire" flight control system. The wiring is routed through the interior of the fuselage. A backup system, consisting of cables and pulleys, also routed through the interior of the fuselage structure, was provided for in case of failure of the primary system.
I.e., this aircraft manufacturer, and others, tried to solve the failure of a primary flight control system by installing a complete, secondary, backup system. The backup system was either a separate system using the same components, or a system using different components. However, nothing was done to protect these two internal control systems from structural damage that might occur due to accidents within the fuselage. Since all backup systems transmit pilot-generated forces to their respective destination flight control surfaces or engines via physical components in the inside of the aircraft, they are susceptible to failure due to structural damage, e.g., by an in-flight engine explosion, an air-to-air collision, terrorist bombs, and military combat. Such structural damage can sever any of these internal physical control systems, thus causing the pilot to lose control over these external flight control surfaces.
E.g., with the hydraulic system, a break in the hydraulic lines can cause a loss of hydraulic fluid, and thus a loss in operating pressure. This loss of operating pressure can also occur if an engine-driven hydraulic pump becomes inoperative due to an engine failure. With no hydraulic pressure, the pilot can no longer operate the external flight control surfaces and the airplane is out of control.
The following incidents were published in the National Transportation and Safety Board Reporter for the year 1989, Vol. 7, Numbers 7 and 9, dated July and Sept. 1989.
Apr. 15, 1988: A Horizon Air, Flight 2658, a DHC-8 aircraft had an engine fire and engine shut down on final approach. After landing, all flight controls, operated hydraulically, were inoperative, causing the aircraft to impact various ground objects. The NTSB concluded that the engine fire had caused "hydraulic line burn-through that in turn caused a total loss of airplane control."
Jul. 19, 1989: United Airlines Flight 232, a DC-10 wide-body, three-engine jet aircraft sustained an engine failure in the center engine located internally in the vertical stabilizer area. Part of the engine exploded and engine fragments severed both the primary hydraulic flight control system and the backup auxiliary system and nearly all hydraulic fluid in the control systems was lost. The pilot of the aircraft had only slight control of the aircraft using differential power in the remaining two engines. The pilot was able to crash land the airplane at the Sioux City, Iowa airport, but in the process loss of life occurred.
Another accident occurred on Aug. 12, 1985 when a Japanese airliner, a Boeing 747, suffered a structural failure on a flight within Japan. The aircraft suffered an explosive decompression when the rear pressure bulkhead ruptured and knocked off part of the vertical stabilizer and rudder. This caused the loss of hydraulic fluid, which meant loss of use of all flight controls. For a lengthy period of time, the pilots were able to control the aircraft by varying the thrust of the engines, keeping the airplane in the air for sometime. Eventually they lost control of the aircraft; it crashed and of 524 persons on board 520 perished.
Another accident occurred on Sept. 22, 1981 when an Eastern Air Lines L-1011, a wide-body, three-engine aircraft, suffered an engine failure in its center engine, causing the loss of all hydraulic control systems.
The National Transportation and Safety Board has officially verified that loss of hydraulic pressure was the cause of all four of the preceding accidents.
There have been other aircraft accidents where the complete cause of the accident has not been determined. Pan American flight 103 departed London on Dec. 21, 1988 and was on course over Lockerbie Scotland when a terrorist bomb exploded. The bomb was the primary cause of the accident, but the airplane may have gone down because the flight control system was severed. We do know that other bombs have exploded on aircraft where damage was confined to certain non-critical areas and did not cause the loss of the airplane.
On Jun. 8, 1992 another unexplained accident occurred to a Boeing 737 of Compania Panama De Aviacion COPA Air Lines, a Panamanian airline. Their Flight 201 went down in the jungles of Panama near the Colombian border. The flight recorder was only able to tell the authorities that the aircraft suddenly banked to one side, then to the other side as it became inverted and went nose down into the ground. The official determination was that all flight control was lost for an unexplained reason.
At least one attempt has been proposed to solve this problem. U.S. Pat. No. 5,330,131, dated Jul. 19, 1994 to Frank Burcham et al., and the Dec. 1993 issue of Popular Science magazine, pages 22 and 72, disclose a backup flight control system using differential engine thrust on both sides of the aircraft. This was developed at NASA's Ames-Dryden Flight Research Facility, Edwards Air Force Base, Edwards, Calif. to provide a means of control in case an aircraft lost its flight control system. This new engines-only backup control system uses a multi-engine aircraft's engines for controlling its flight path by varying the thrust of each engine. The engines are controlled by electronic signals from the pilot. These signals are in turn modified by an electronic controller so as to change to the correct output power of the engines on each side of the aircraft. These electronic control signals are transmitted by wire through the internal structure of the aircraft. However, this solution does not solve the loss of control problems in those incidents where all engine power is lost on one side of the aircraft, or when the transmitting wires are severed inside the internal fuselage structure. This type of engine-only backup control system must rely on normal engine power on both sides of a multi-engine aircraft and is not operable if engine power is lost on one side of the aircraft. Further, this system is not feasible on a single engine aircraft.